/Engine/Plugins/Experimental/GeometryScripting/Source/GeometryScriptingCore/Public/GeometryScript/MeshMaterialFunctions.h
Returns the current Material ID for a Triangle. If the mesh does not have Material IDs enabled or if the Triangle ID is not an element of the mesh, the value 0 will be returned and bIsValidTriangle will be false.
static int32 GetTriangleMaterialID ( [UDynamicMesh](API\Runtime\GeometryFramework\UDynamicMesh) * TargetMesh, int TriangleID, bool & bIsValidTriangle )